1. Field of the Invention
The present invention relate to a magnetic resonance imaging (MRI) apparatus for imaging a magnetic resonance signal generated from a subject, and particularly relates to a constitution thereof for preventing the nerve of a subject, caused by a change of an applied magnetic field over time, from being stimulated.
2. Description of the Related Art
A magnetic resonance imaging apparatus is an apparatus in which chemical and physical microscopic information of a substance is imaged, or a chemical shift spectrum thereof is observed by utilizing a phenomenon of resonantly absorbing an energy of a high frequency magnetic field rotated by a specific frequency when a group of nuclei having an inherent magnetic moment is placed in a uniform magnetostatic field.
In diagnosis using the magnetic resonance imaging apparatus, a gradient magnetic field for executing phase encoding and frequency encoding is swiftly applied and, thereafter, application/stop of a gradient magnetic field B are repeatedly executed at high speed in accordance with a predetermined sequence. During this sequence, an eddy current can be generated in the subject in proportion to a rate dB/dt of a change in a magnetic field over time which can stimulate the nerve of the subject.
In order to prevent the nerve from being stimulated in this way, according to the IEC (International Electrotechnical Commission), standard of the IEC requires that no problem is posed by the imaging sequence when it is confirmed that the nerve is not stimulated with regard to a specific number of persons by unspecified volunteers. Further the magnetic resonance imaging apparatus is controlled to constitute a sequence in conformity with the standard by checking a gradient magnetic field by software in the user interface.
However, a magnetic resonance imaging apparatus of a related art poses, for example, a problem described below. That is, even when the confirmation is carried out with regard to a specified number of persons in conformity with the IEC standard, it is not that no problem is always posed to all the subjects. Further, in related arts, there are technologies of determining an imaging position by changing the imaging position while observing a real image by using a real time locator as in JP-B-5-42126 and JP-A-4-71535. However, according to the technologies, even when all of the gradient magnetic fields B applied by softwares can be confirmed, a final output cannot be confirmed. Therefore, there is a case of bringing about omission of check. Meanwhile, when there is constituted a sequence which is safe for all the cases, there is a concern that the standard becomes severe more than necessary, which is not practical.
The present invention has been carried out in view of the above-described situation and it is an object thereof to provide a magnetic resonance imaging apparatus capable of realizing to apply a safe magnetic field at high speed and without omission of check. sum